A lithium ion battery has been conventionally used as a power supply for a portable electronic apparatus. The lithium ion battery is generally configured by disposing, in an electrolyte solution, a positive electrode, a negative electrode, and a separator. The positive electrode is formed by applying lithium cobaltate or lithium manganate to the surface of an aluminum foil. The negative electrode is formed by applying carbon to the surface of a copper foil. The separator separates the positive electrode and the negative electrode to prevent a short circuit between them.
When the lithium ion battery is charged, lithium ions are released from the positive electrode and move to the negative electrode. In contrast, when the lithium ion battery is discharged, lithium ions are released from the negative electrode and move to the positive electrode. In this manner, the lithium ion battery is charged and discharged. Therefore, it is necessary that the separator used in the lithium ion battery can allow lithium ions to well permeate there.
When the lithium ion battery is repeatedly charged and discharged, dendrites (dendrite crystals) of lithium are generated on the end surface of the negative electrode. The dendrites break through the separator to cause a very small internal short circuit (dendrite short circuit) between the positive electrode and the negative electrode, thereby remarkably deteriorating the battery capacity.
In order to improve the safety of the lithium ion battery, an olefinic resin porous film which mainly includes polyethylene is used as the separator. This is because when the lithium ion battery generates abnormal heat due to a short circuit or the like, polyethylene constituting the porous film melts at a temperature region of about 130° C. to block a porous structure (shut down), and as a result, the abnormal heat is stopped. Thus, the safety can be secured.
In recent years, an increase in the output of a large-type battery such as a lithium ion battery for automobiles has proceeded, and further, a temperature may be rapidly increased to higher than 130° C. For this reason, a shutdown function is not necessarily required, and the heat resistance of the lithium ion battery is considered as an important factor. In order to increase the output of the lithium ion battery, a decrease in the resistance at which lithium ions pass through the separator is required. Therefore, high gas permeability is required for the separator. Further, in the large-type lithium ion battery, it is important to ensure a long lifetime and long-term safety.
Various separators using a porous film made of polypropylene having a high heat resistance have been proposed. Patent Literature 1 has proposed a method for producing a polypropylene microporous film. The method includes melting and extruding a composition containing polypropylene, a polymer having a melt crystallization temperature higher than that of polypropylene, and a β-crystal nucleating agent, for example, to mold the composition into a sheet at high temperature, and stretching the sheet at least uniaxially.
However, a polypropylene microporous film obtained by the above method for producing a polypropylene microporous film has a low degree of gas permeability and insufficient lithium ion permeability. Therefore, it is difficult that the polypropylene microporous film is used for a lithium ion battery requiring a high output.
Patent Literature 2 has proposed a multilayered porous membrane. The multilayered porous membrane comprises a polyolefin resin porous membrane and a porous layer which is formed on at least one side of the polyolefin resin porous membrane, contains an inorganic filler or a resin having a melting point and/or a glass transition temperature of 180° C. or higher, and has a thickness of 0.2 μm to 100 μm. Furthermore, the multilayered porous membrane has a degree of gas permeability of 1 to 650 sec./100 cc. However, the above multilayered porous membrane has insufficient lithium ion permeability, and therefore it is difficult that the multilayered porous membrane is used for the lithium ion battery requiring a high output.
Patent Literature 3 has proposed a non-aqueous electrolyte solution battery having a negative electrode made of light metal, a separator impregnated with a non-aqueous electrolyte solution, and a positive electrode, wherein a polyethylene fine powder is previously bonded onto the separator. Further, a high heat-resistant non-woven polypropylene fabric suitable for the application of a high output battery is used as the separator.
However, the separator has a pore size as large as several micrometers, and therefore it is expected that a small short circuit easily occurs. In addition to problems such as insufficient lifetime and long-term safety, the separator has a disadvantage of difficulty of formation of a thinner film since the non-woven fabric is used.